It's a small decoration tree with a push button and a small keyboard. If you press the button it starts playing jingle bells and lights up the appropriate key on the piano keyboard. On the backside of the tree there is the battery, a speaker, an ATmega8 to control everything and some simple electronics to put stuff together. I described everything step by step here:
http://www.snej.de/2012/11/jingle-bells-playing-light-up-piano.html